In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, November 1999). Performance may be improved if both the transmitter and the receiver for a base station sector are equipped with multiple antennas (e.g., an sector antenna array) to provide a multiple-input multiple-output (MIMO) communication channel(s) for the base station sector. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink from a base station transmitting from a sector antenna array over a MIMO channel to a wireless terminal in the sector, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from the base station sector antenna array for the sector. Stated in other words, multiple symbol streams may be transmitted from the base station sector antenna array for the sector to the wireless terminal over the same downlink transmission time interval (TTI) and/or time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station sector transmitting from the same sector antenna array to the same wireless terminal, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station sector antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station sector antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to transmit diversity gain.
Four layer MIMO transmission (4Tx) schemes are proposed for High-Speed-Downlink-Packet-Access (HSDPA) within Third Generation Partnership Project (3GPP) standardization as disclosed, for example, in 3GPP RP-111393 (“New WI: Four Branch MIMO Transmission For HSDPA,” 3GPP TSG-RAN meeting #53, Fukuoka, Japan, Sep. 13-16, 2011) and 3GPP R1-111763 (“4-branch MIMO for HSDPA,” 3GPP TAG RAN WG1 Meeting #5, Barcelona, Spain, May 9-13, 2011), the disclosures of both of which are hereby incorporated herein in their entireties by reference. Accordingly, up to 4 layers of information/data may be transmitted in parallel using a same TTI/TFRE when using 4-branch MIMO transmission.
The number of codewords supported by 4 branch MIMO is one issue regarding implementation of 4 branch MIMO. Contributions discussing possible configurations for 4×4 MIMO include 3GPP R1-113432 (“4×4 DL MIMO HS-DPCCH Design,” 3GPP TSG RAN WG1 Meeting #66bis, Zhuhai, P. R. China, Oct. 10-14, 2011), 3GPP R1-112979 (“Discussion on 4-Branch MIMO Design Options,” 3GPP TSG-RAN WG1 Meeting #66bis, Zhuhia, China, Oct. 10-14, 2011), and 3GPP R1-113360 (“Codeword to Layer Mapping Alternatives For DL 4 Branch MIMO,” 3GPP TSG-RAN WG1 Meeting #66bis, Zhuhia, China, Oct. 10-14, 2011), the disclosures of which are hereby incorporated herein in their entireties by reference.
Among proposed configurations for 4×4 MIMO, configurations for 4 codeword 4×4 MIMO and 2 codeword 4×4 MIMO are likely candidates for further investigation. In 3GPP R1-114290 (“Number of Supported Codewords for 4-branch MIMO,” 3GPP TSG RAN WG1 Meeting #67, San Francisco, USA, Nov. 14-18, 2011), a 2 codeword 4 branch MIMO is proposed for standardization, and the disclosure of 3GPP R1-114290 is hereby incorporated herein in its entirety by reference.
Channel quality information for a 4×4 MIMO system, however, may be difficult to compute and/or report.